1. Field of the Invention
This invention relates to an apparatus for servicing a steam generator and, more particularly, to an apparatus for remotely inspecting and cleaning areas between the tubes on a tube sheet in a nuclear steam generator.
2. Description of the Prior Art
In a pressurized water nuclear powered electric generating system, the heat generated by the nuclear reactor is absorbed by a primary coolant that circulates through the reactor core and is utilized to generate steam in a steam generator. The steam generator typically is an upright cylindrical pressure vessel with hemispherical end sections. Such a generator typically comprises an outer vertically oriented shell, a horizontal plate called a tube sheet adjacent the lower end of the shell, a bundle of vertical U-shaped tubes supported by the tube sheet, and a wrapper barrel inside of the outer shell surrounding the tubes and extending from the upper portion of the shell downwardly to a predetermined point above the tube sheet. The wrapper barrel forms a narrow annulus inside the shell and generally extends down to a point six inches above the tube sheet. The outer cylindrical shell is provided with openings of limited size called handholes which are located 18 inches above the tube sheet. These handhole openings are covered during operation of the steam generator but may be opened when the generator is shut down to permit access to the area inside for maintenance purposes.
Occasionally, during maintenance inside the steam generator, objects such as bolts are dropped and have to be removed. Due to the limited space within the generator, the annulus between barrel wrapper and the shell generally is only two inches wide and the space beneath the bottom of the barrel wrapper is only six inches high. The bundle of tubes supported by the tube sheet also are very closely spaced together. Therefore, it is difficult to insert a device into the generator and to pick up objects between the pipes on the tube sheet.
While certain inspection and maintenance devices for steam generators are known, none of these devices are adapted to be able to search between the tubes of a steam generator and also retrieve loose objects in the narrow spaces between the tubes. For example, Creek et al. U.S. Pat. No. 4,515,747 discloses a remotely operated maintenance and inspection device which includes a wheeled transporter which is inserted through a handhole opening in the shell of a steam generator and positioned on the tube sheet between the tube bundle and the inner wall of the shell. A cable is then attached to each end of the transporter and extends around the bundle of tubes and out through the handhole opening whereby suitable power means may be utilized to move the transporter around the periphery of the tube bundle. A TV camera may be mounted on the transporter to inspect the tubes of the bundle. A sludge lance also may be mounted upon the transporter to break up and liquify deposits at the base of the tubes of the bundle. The Creek et al. device, however, does not provide any means for employing a device which can be manipulated through the bundle of tubes to pick up objects on the tube sheet. Moreover, since the transporter and its associated cable contact the tubes of the tube bundle, there is a risk that the tubes might be damaged during operation of the device.
Another type of prior art steam generator inspection device is disclosed in McShane et al. U.S. Pat. No. 4,355,536. This device basically comprises a probe mounted on the end of a ribbon-like probe carrier which is fed into the shell of a steam generator through the handhole opening and deflected by a carriage placed in alignment with a lane of tubes whereby the probe may measure the accumulation of sludge adjacent the bottom of the tubes. No means is provided on the device, however, for picking up and retrieving objects from the tube sheet.
Accordingly, a need still exists for a device capable of inspecting or searching the tube sheet in the areas between the vertical tubes and which also is adapted to pick up any loose objects between the tubes on the tube sheet.